<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<input type="text" id="i1">
<input type="text" id="i2">
<input type="button" value="加" onclick="f(1)">
<input type="button" value="减" onclick="f(2)">
<input type="button" value="乘" onclick="f(3)">
<input type="button" value="除" onclick="f(4)">
<div>结果:</div>
<script>
  let n1 = document.querySelector("#i1")
  let n2 = document.querySelector("#i2")
  let d = document.querySelector("div")
  function f(value) {
    if(isNaN(n1.value)||isNaN(n2.value)){
      d.innerText = "输入有误!"
      return;
    }
    switch (value) {
        case 1:
            d.innerText = "结果:" + (parseFloat(n1.value) + parseFloat(n2.value));
            break;
        case 2:
            d.innerText = "结果:" + (n1.value - n2.value)
            break;
        case 3:
            d.innerText = "结果:" + n1.value * n2.value;
            break;
        case 4:
            d.innerText = "结果:" + n1.value / n2.value
            break;
    }
  }
</script>
</body>
</html>